In accordance with the progress of hydration reaction of cement, concrete gradually loses flow with the lapse of time after mixing. This suggests that concrete loses applicability and workability with the lapse of time. With respect to pumping of concrete through a feed conduit, a problem arises if the pumping operation is interrupted on account of an operator break or a change of course and restarted later. Since the concrete in the feed conduit reduces its flow during the quiescent time, the pumping pressure sharply rises at the restart of pumping. In case of a further flow reduction, the feed conduit can be clogged with the concrete.
In the prior art, admixtures are added to hydraulic concrete compositions for improving their flow. Known admixtures are water-reducing agents and high-range water-reducing agents including lignin sulfonic acid base water-reducing agents, polycarboxylic acid base water-reducing agents, naphthalene base water-reducing agents (e.g., naphthalenesulfonic acid/formaldehyde condensate salts), and melamine base water-reducing agents (e.g., melamine sulfonic acid/formaldehyde condensate salts). Although these admixtures are effective for improving flow, they suffer from a substantial slump loss, i.e., loss of flow with time.
For preventing physical properties of freshly mixed concrete from changing with time, it is a common practice to select the type and amount of a slump retainer, set retarder and set accelerator. For example, Patent Document 1 discloses a combination of a dispersant with a set accelerator such as calcium nitrite. Patent Documents 2 and 3 disclose a combination of a slump retainer such as oxycarboxylic acids or salts thereof with a set retarder such as succharides or succharide alcohols.
The method of Patent Document 1 is difficult to meet both segregation resistance and slump loss, since the effect is largely affected by the ratio of two admixtures. The methods of Patent Documents 2 and 3 provide concrete with a low slump loss after 60 minutes of intermittent mixing. However, once the working is interrupted, pumping at the restart of working may become difficult.